Field types of a rails model

Hi,

I don't understand something about the types of the fields/attributes
of rails models.

In one of my models, InvestmentAdmittanceTransaction, I have a field
called client_new_participations. It's a numeric value (I validate it
with validates_numericality_of, and it's a numeric value in
postgresql). In the validate method of the model, the attribute is a
string.

In another model, InvestmentProcurementTransaction, I have a field
called fund_cash_payment. It's identical to the above one in type,
i.e. a numeric. But, when using it in the validate method, it's a
BigDecimal.

Why are those fields different types in each model, even though the
situation is exactly the same?